The present invention relates to mechanisms that protect equipment from damage due to electrical faults and short circuits; and particularly to such devices which electronically monitor performance of the electric equipment and take protective action in the event of a fault, short or overload.
It is important that electrical apparatus be protected from damage when electrical failures occur. For example, conventional fuses and electromechanical circuit breakers are commonly employed to disconnect equipment from an electrical supply upon detection of excessive current when a short circuit occurs. Nevertheless, these conventional protection devices are relatively slow in disconnecting the current flow to the apparatus being protected. As a consequence, enough excessive electrical current can flow into the equipment to cause damage during a fault.
Furthermore, various electrical apparatus require different response characteristics for the protection device. For example, electronic equipment may draw a substantially constant current level from initial start-up through a normal shut-down and be very intolerant of excessive current levels of even short duration. The protection device for such equipment has to respond very quickly to even relatively small over current conditions. Other types of electrical equipment draw large instantaneous current levels at certain times, such as upon start-up, in comparison to the current level drawn during remainder of their operation. Thus, a circuit protection device that responds too rapidly to an high current condition may inadvertently shut-off current to the equipment during normally occurring events. As a consequence, the protection device for this type of equipment must respond in a manner that tolerates brief high currents. The manner in which a protection device responds to over currents is referred to as the trip response characteristic or trip curve, and has to be matched to the particular type of electrical apparatus being protected.
This usually means that a manufacturer of protection devices must design, manufacture and stock in inventory, a large variety of protection devices that have different trip response characteristics in terms of current level and duration. Thus it is desirable to provide a basic configuration of a protection device which can be customized easily with different trip response characteristics.
An apparatus for protecting an electrical load from excessive current employs a semiconductor switch to connect the electrical load to a source of current. A current sensor is coupled in series with the semiconductor switch and produces a sensor signal that indicates the magnitude of current flowing to the electrical load.
A control circuit is connected to the current sensor and the semiconductor switch. The control circuit responds to the sensor signal by producing a control signal that is applied to a control input of the semiconductor switch. In a first mode of operation when the magnitude of current is less than a first threshold, the control circuit maintains the semiconductor switch in a continuous conductive state. When the magnitude of current is greater than the first threshold and less than a second threshold, the control circuit in a second mode of operation renders the semiconductor switch non-conductive after a predefined period of time. In a third mode of operation when the magnitude of current is greater than the second threshold, the semiconductor switch is alternately pulsed conductive and non-conductive by the control circuit to apply an average current through the load that is within an acceptable level wherein damage does not occur.